Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are ejected onto the print media such as paper or transparency film during a printing operation. These drops of ink combine on the print media to form the text and images perceived by the human eye. A key component of the printer for the deposition of these ink drops is the printhead, which is connected to an ink supply and contains an arrangement of nozzles and a mechanism which allows ink drops to be controllably ejected from each individual nozzle. One or more printheads may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. Inkjet printers may use a number of different ink colors. An inkjet printer frequently can accommodate two to four print cartridges. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over a given location, called a pixel, on the media which is to be printed. Each print cartridge typically has an arrangement of printhead nozzles through which the ink is controllably ejected onto the print media, and thus a certain width of the media corresponding to the layout of the nozzles on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, the entire printable area of the media can be printed.
To avoid printing defects that adversely affect print quality, such as bleeding of one color area into another, bands of unprinted color, and warping or wrinkling of the print media, most printers do not print all drops of all ink colors in all pixel locations addressable in one single forward or rearward scan and then advance the media by the height of the nozzle arrangement. Rather, multiple scans are used to fully ink any given pixel location, with the media advanced after each scan by a portion of the height of the nozzle arrangement. Only a fraction of the total ink required in each section of the image is laid down in any single scan. Areas left unprinted are filled in by one or more later passes. For each scan, a group of printmasks, or patterns used in printing a section of nozzles for particular printheads, determine which drops of which inks are deposited at each pixel location. The printmasks mix up the nozzles used on different scans in such a way as to reduce these undesirable visible printing defects by controlling the amount and location of liquid ink that is on the page at any given time. In other words, the printmasks govern the final step of the printing process, determining in which pass or passes each pixel location will be addressed, after prior rendition steps have already determined (from the digital data for the image to be printed) the required color and intensity for each pixel location.
Some printers have a mode in which they only deposit ink while scanning in one direction; for example, in the forward scan but not the rearward scan. This print mode is known as unidirectional printing. Conversely, some printers have a mode in which they deposit ink while scanning in both directions, known as bidirectional printing. Bidirectional printing has the advantage of reduced printing time, because it avoids the wasted motion of moving the carriage in the rearward direction without printing required for unidirectional printing. However, bidirectional printing can introduce a print quality defect known as color hue shift, as explained below.
Each printhead deposits ink of a different color on the media. In one commonly used arrangement, the inks are the primary subtractive colors magenta, cyan, and yellow. Alternatively the printer can use more than three color inks, some of which are lighter and darker versions of a given color shade. In some embodiments, there is also a black ink, drops of which may be used during color printing to form the darker shades of colors. Drops of the color inks can be combined in the same pixels to form a range of perceived colors to the human eye. For example, superimposing drops of magenta and cyan in the same pixel location produces a blue composite color pixel. If there were no interactions between the ink and the media, the order in which the magenta and cyan ink drops are deposited on the print media does not matter. However, the ink and the media do interact, and thus the color shade or hue that is perceived by the observer depends on the order in which the drops of the different color inks are deposited on the media.
It is the arrangement of the print cartridges in the carriage that causes the differing order of drop deposition that creates a hue shift between different regions of what should be the same color in bidirectional printing. For example, in a typical inkjet printer cyan, black, magenta, and yellow printheads are aligned side-by-side in the carriage. When the carriage sweeps in the forward direction, the yellow printhead will pass over a particular pixel location on the print media first, followed by the magenta, then the black, and finally the cyan. If a blue color is to be printed in the pixel location, the magenta drop would be deposited before the cyan. In contrast, when the carriage sweeps in the rearward direction, the cyan printhead will pass over a particular pixel location on the print media first, followed by the black, magenta, then yellow printheads. If a blue color is to be printed in the pixel location, the cyan drop would be deposited before the magenta.
The hue shift problem is most noticeable when a contiguous area of pixels spanning more than one scan of the cartridges over the media is printed with the same composite color, in particular a deep shade of two primaries such as blue or green (green is produced by combined cyan and yellow drops). This printing operation produces perceptibly different bands of color shades between bands generated by printing in the forward direction followed by the rearward direction, and bands generated by printing in the rearward direction followed by the forward direction.
There have been two general approaches to reducing or eliminating the problems of hue shift in bidirectional color inkjet printing. These approaches usually improve print quality at the cost of reduced throughput (eg. slower printing speed) or increased printer cost or complexity. The first general approach is to eliminate hue shift by depositing the color ink drops in all pixel locations on the print media in the same order. In one prior art implementation of this solution, the printheads are not in-line but rather are offset from each other in the media advance direction by the full height of the nozzle array, so that only drops of one color can be deposited in a given pixel location on a single forward or rearward scan of the carriage. However, this implementation results in a wide print zone requiring extra cost and complexity to hold the paper flat and without relative motion. An alternate implementation uses in-line printheads arranged in the carriage not only in a cyan-magenta-yellow order but also in a yellow-magenta-cyan order, with the first group used in the forward scan direction and the second group used in the rearward scan direction to ensure a consistent order of drop deposition. However, this solution also adds cost and complexity to the plumbing of the ink supply or to the print cartridges.
A third prior art implementation that is the subject of the above-mentioned Majette application requires a complex medium advance mechanism that can also reverse, instead of merely advancing, the media movement between scans in order to always lay the drops of different color inks down in the same order. A variation of this implementation also disclosed in Majette masks off a top group of nozzles of each printhead in a forward scan and a bottom group of nozzles in a rearward scan to achieve the same effect without the need for a complex medium advance mechanism, but at a cost of reduced throughput since the reduced effective width of the scan path requires more scans to fully ink all pixel locations on a page. A similar throughput reduction results from a prior art implementation that eliminates hue shift by scanning in both the forward and rearward directions before advancing a portion of the nozzle height.
The second general prior art solution to solving the hue shift in bidirectional color inkjet printing is to reduce the hue shift to make it less perceptible to the human eye. Since the hue shift is most noticeable between bands, one implementation disclosed in the above-mentioned Majette application advances the media in uneven portions; for example, by a larger portion of the width of the nozzle arrangement in the forward direction, and a smaller portion in the rearward direction. This results in one wider band having one hue and one narrower band having the other hue, thus reducing the perceivability of the shift. However, the hue of each bands is still the same as it previously was, and the print quality improvement is limited by how much the band can be narrowed; if it becomes too narrow, or if a printhead defect affects a region of nozzles, medium-advance defects such as bands of unprinted color will appear. An alternative implementation which uses multiple drops of each color ink to produce the composite color diminishes the bidirectional color shift by averaging the order in which the colors are printed. For example, if two drops of each color are used to form the composite color, a band printed in the forward direction before the rearward direction will have pixels in which the bottom-to-top order of drops will be magenta, cyan, cyan, and magenta. Conversely, in a band printed in the rearward direction before the forward direction, the order will be cyan, magenta, magenta, and cyan. However, in many cases this is insufficient to adequately reduce the hue shift effect of ink/media interactions.
Accordingly, the need still exists for a color inkjet printer having a print mode that can significantly reduce the problem of hue shift without compromising throughput and without requiring extra cost or complexity in the printing mechanism.